Production of electrical resistor elements



1962 E. M. DITTMAN ETAL PRODUCTION OF ELECTRICAL RESISTOR ELEMENTS Filed Jan. 11, 1957 United States Patent 3,061,501 PRODUCTION OF ELECTRICAL RESISTOR ELEMENTS Emil M. Dittman, Joseph J. Coleman, and Ernest F.

Steinhoff, Freeport, Ill., assignors, by mesne assignments, to Serve], Inc., Evansville, 11111., a corporation of Delaware Filed Jan. 11, 1957, Ser. No. 633,739 1 Claim. (Cl. 156-250) This invention relates'to production of electrical resistor elements more particularly from sheets or strips by a markedly simple and low-cost procedure and in the form of a sandwich-like structure.

In the electronic and automation equipment fields especially and particularly by reason of the growing trend toward miniaturization inthese fields, an increasing demand is apparent for a low cost resistor element which lends itself to resistance control by simple and inexpensive expedients that otherwise retain the same steps of manufacture and structural assembly. v

The invention will be understood by reference to the following description, taken together with the accompanying drawing, of illustrative embodiments thereof, and in which drawing- FIGURE 1 is a perspective View, partially broken away for purposes of description, of an integral laminated sheet or strip embodying the present invention and from which portions or sections may be cut to provide individual resistor elements;

FIGURE 2 is a cross-sectional view of one such resistor element with wire terminal connections;

FIGURE 3 shows a strip of laminate from which individual resistor elements may be cut or sliced; and

FIGURE 4 shows a modification.

Referring in detail to the illustrative construction shown in the drawing and steps of procedure in accordance with the present invention, the sandwich-like structure of FIGS. 1 and '2 is prepared by providing a layer of conductive foil or the like 11, preferably a copper foil, superimposing or spreading thereon an electrically resistant or semi-conductive adhesive layer 12, and then superimposing on the layer 12, to complete the sandwich, another electrically conductive foil 13 similar to the layer 11. The foil layers 11 and 13 are each preferably of the order of 0.005 inch thick.

The layer 12 is constituted of a binder of a low or non-conductive adhesive substance impregnated with particles 12a of electrically conductive material such as graphite. The term semi-conductive is used herein in its broader sense, meaning partly conductive and partly resistive, and not in its technical sense. The material of the layer 12 is non-thermoplastic. Layer 12 may have a thickness of about 0.012 inch.

An example of the manufacture of the adhesive layer 12 is as follows: First a slurry of the adhesive composition is prepared. A rubber phenolic adhesive is preferred. Such available adhesive is Plastilock No. 604 made and sold by the B. F. Goodrich Company, which is a non-thermoplastic, water resistant and aromatic oil resistant adhesive comprised of a mixture of nitrile rubber and phenolic resin, compounded with curatives. This adhesive has been found to have exceptional bond strength and to stand up well under the moderate temperatures to which the resistors are subjected. Great Lakes artificial graphite may be employed as the electrically conductive material. To these ingredients are added an organic solvent, for example, methyl ethyl ketone. Other electrically conductive components could be used, such as acetylene black or other carbon blacks.

The slurry may be prepared by mixing 77 parts of Plastilock No. 604 containing 71 percent solids with 23 "ice parts of graphite (-200 mesh) and a sufficient amount of methyl ethyl ketone to obtain a slurry having a viscosity of 15 centipoises. Such a mixture is then subjected to a milling treatment, e.g. in a ball mill. After the milling operation is completed the slurry is screened and ready for use.

Compositions according to the present invention may vary considerably from those disclosed above. In general, adhesive compositions are suitable which result in a cured semi-conductive adhesive layer comprised of from approximately to percent by weight of rubber and resin together, and 20 to 30 percent by weight of graphite.

A thermosetting composition according to the present invention may also be prepared by mixing together an uncured thermosetting phenolic resin, either in a liquid form or dissolved in a solvent, together with carbon such as in the form of graphite. This composition may be applied and cured in much the same way as the previously described composition.

The laminated sheet or strip may be prepared by coating one side of the layer 11, that is the upper side thereof, with the adhesive and then coating one side of the layer 13, that is the lower side thereof in FIG. '1, with the adhesive, and allowing the coatings to dry.

Bonding of the layers is accomplished by heating under pressure. For example, a pressure of 40 pounds per square inch and temperature of 400 F. may be used, and the assembly is preferably cooled while still under pressure.

As an alternative procedure, layer 11 may be coated with a layer of adhesive and layer 13 may be provided by spraying copper thereon.

Thereafter, as indicated in FIG. 3, the laminate sheet or strip comprising the layers 11, 12 and 13 can be cut or sliced as suggested by dotted lines A to produce individual resistor elements, it being understood that the distance apart of dotted lines A may be varied according to the sizes desired.

A section of the composite layer 11--12-13, cut off as previously indicated, may be employed, as indicated at 14 (FIG. 2), one wire terminal 15 being suitably connected to the layer 13 as at 16 and another wire terminal 17 connected to the layer 11 as at 18. The connection of the lead conductors may be made as by soldering or welding.

The resistance of the element per square inch may be varied by increasing or decreasing the percentage of graphite in the layer 12. Other ways of controlling or varying the resistance value of the element are by varying the area of the section selected or the thickness of the layer 12.

In order to avoid any possibility of a short circuit between the metal layers 11 and 13 the modification shown in FIG. 4 may be employed in which the metal conductors 11 and .13 are separated not only by the semi-conductor layers 1% and of adhesive material as previously described, but the layers 12b and 12c are in turn separated by a layer of light flexible insulating recticulated or foraminous material such as cheesecloth, or other such net-like substance.

In forming the structure of FIG. 4, the two metal sheets 11 and 13 are precoated with the adhesive layers 12b and 12c respectively and these coatings allowed to dry. Then a second coat of adhesive is applied to the coating 12b on the upper side of sheet 11. Before the second coat is dry the cheesecloth 19 is pressed into it and another coating is applied on top of the cheesecloth, and these composite coatings allowed to dry. Sheet 13 with its 0 undercoating of dry adhesive is then placed on top of the last coating and bonded to the assembly by pressure and heat.

s,os1,501

A loose weave cheesecloth is preferably employed, of say 30 x 30 threads per square inch, so that the semi-conductive adhesive will flow through it. The cheesecloth layer may have a thickness of say .005 inch and the composite semi-conductive layer 12b19-12ca thickness of say .025 inch.

It is to be understood that when a resistor element is herein referred to it is intended to refer to an element which may be used also as a heating element, since electrical current flowing through the semi-conductive material 12 creates heat.

The invention having been described, what is here desired to be covered by Letters Patent is set forth in the appended claim, it being understood that such changes may be made, including modifications or additions, as fall Within the scope of said claim without departing from the invention.

What is here claimed is:

The method of making laminated resistor elements which comprises the steps of coating an extended area of a conductive foil layer with a semi-conductive composition consisting essentially of a thermosetting resin having carbon particles dispersed therein, imbedding a fabric interlayer in said composition layer, covering the other side of said interlayer and said semi-conductive composition with a second conductive foil, curing said thermosetting resin to form a laminated resistor stock sheet having substantially uniform resistivity between said foil layers throughout the area of said sheet, and cutting sections of the laminated sheet thus formed into individual resistor elements of the desired resistance value and power handling capacity.

References Cited in the file of this patent UNITED STATES PATENTS 2,120,930 Cooper June 14, 1938 2,218,373 Alexander Oct. 15, 1940 2,358,211 Christensen et a1 Sept. 12, 1944 2,361,438 Turner Oct. 3 1, 1944 2,412,201 Brennan Dec. 10, 1946 2,444,314 Ruggieri June 29, 1948 2,491,320 Koontz Dec. 1 3, 1949 2,597,741 Macey May 20, ,1952 2,664,044 Dalton Dec. 29, 1953 2,683,673 Silversher July 13, 1954 2,688,576 Ryan et al Sept. 7, 1954 2,692,321 Hicks Oct. 19, 1954 2,708,354 Wainer Mar. 1, 1955 OTHER REFERENCES Modern Plastics, August 1951, pages 99-111. 

